1. Field of the Invention
This invention relates to third order modulation distortion, and, more specifically, to techniques for reducing or eliminating such distortion.
2. Background
In a quadrature modulator 1, illustrated in FIG. 1a, I and Q components of a baseband signal are respectively mixed by mixers 2 and 3 with I and Q components of a local oscillator signal, LOI and LOQ, and then combined by combiner 4 to form output signal 5. The I and Q components of the baseband signal are assumed to be in quadrature, i.e., out of phase by 90xc2x0. Similarly, the I and Q compopnents of the local oscillator signal are assumed to be at the same amplitude, and are also assumed to be in quadrature.
In applications in which the modulator 1 is part of a GSM wireless transmitter, a problem occurs when third order modulation distortion is introduced by modulator 1 in the output signal 5. The problem is that any third order product in the transmit signal must be less than xe2x88x9250 dBc in order to meet the stringent spectral requirements of the GSM specification. However, since the transmit signal is ultimately derived from the output of the quadrature modulator, any significant third order distortion in the output of the quadrature modulator will, in current designs, makes it difficult for the transmit signal to satisfy the GSM spectral requirement.
Moreover, simply boosting the linearity of the modulator by increasing the current thereto is not satisfactory since it results in excessive current consumption, and it also does not solve for third order distortion introduced by subsequent stages.
Consequently, there is a need for a system for and method of reducing third order modulation distortion which is more efficient or effective than conventional approaches.
In accordance with the purpose of the invention as broadly described herein, there is provided a system of reducing third order modulation distortion in which a baseband signal is predistorted by a predistortion circuit, and the predistorted baseband signal is then input to a quadrature modulator, which may be followed by a translational loop to upconvert the output signal from the quadrature modulator to RF frequencies. The predistortion introduced by the predistortion circuit compensates at least in part for third order distortion introduced by the quadrature modulator. The predistortion introduced by the predistortion circuit may be determined during a calibration procedure or may be determined in real-time.
In one implementation, the I and Q components of the baseband signal are predistorted in accordance with the following equations (in which the predistortion which is introduced is represented by the second and third terms in both equations):
I=Axc2x7cos(xcfx86)+C3+xc2x7cos(3xcfx86+xcex83+)+C3xe2x88x92xc2x7cos(xe2x88x923xcfx86+xcex83xe2x88x92)
Q=Axc2x7sin(xcfx86)+C3+xc2x7sin(3xcfx86+xcex83+)+C3xe2x88x92xc2x7sin(xe2x88x923xcfx86+xcex83xe2x88x92)
The ideal output of a modulator with I and Q input signals as given by the foregoing equations is:
s(t)=Axc2x7sin(xcfx89ct+xcfx86)+C3+xc2x7sin(xcfx89ct+3xcfx86+xcex83+)+C3xe2x88x92xc2x7sin(xcfx89ctxe2x88x923xcfx86+xcex83xe2x88x92
For a modulator with 3rd order distortion, a good approximation to the output signal is:
s(t)≅Axc2x7sin(xcfx89ct+xcfx86)+C3+xc2x7sin(xcfx89ct+3xcfx86+xcex83+)+C3xe2x88x92xc2x7sin(xcfx89ctxe2x88x923xcfx86+xcex83xe2x88x92)+A3+xc2x7sin(xcfx89ct+3xcfx86+xcex943+)+A3xe2x88x92xc2x7sin(xcfx89ctxe2x88x923xcfx86+xcex943xe2x88x92)
In this implementation, through a calibration procedure, the predistortion parameters xcex83+, C3+, xcex83xe2x88x92, and C3xe2x88x92 are set as follows:
xcex83+≅xcex943+
C3+≅xe2x88x92A3+
xcex83xe2x88x92≅xcex943xe2x88x92
C3xe2x88x92≅xe2x88x92A3xe2x88x92
Consequently, it can be seen that the 3rd order products in the foregoing equation will be reduced or canceled.
A related method of operation is also provided.